1. Field of the Invention
The present invention relates to seismic survey equipment. In particular, the invention relates to equipment assembly combinations and the logistics of equipment deployment.
2. Description of the Related Art
In principle, a seismic survey represents an analysis of the earth's layered density as indicated by seismic reflections from abrupt density discontinuities at the layer interfaces. The analysis is also influenced by seismic wave propagation velocities respective to the differences in strata density or elasticity. A precisely timed seismic event of known energy magnitude such as the ignition of buried explosives in a shallow borehole or that of a hammer shock propagated against the earth's surface is launched against the earth at a precisely known location and time. Seismic wave reflections of this man-made seismic event are detected by arrays of geophones distributed in an orderly grid over the area of interest. The location of each geophone array is precisely mapped relative to the location of the seismic event. As the seismic wave from the timed event travels out from the source, reflections from that original seismic wave are returned to the surface where they are detected by the geophones. The geophones respond to the receipt of a wave with a corresponding analog electrical signal which is summed with the signals of the other geophones within the array in analog form. The summed analog signals are received from the geophone arrays by data acquisition modules which digitize the analog signal stream for retransmission to a central recording unit as a series of digital data packets. Among the significant data preserved by the data acquisition modules is the amplitude or the strength of the reflected wave and the exact time lapse from the moment the event occurred.
In a single survey, there may be thousands of geophone arrays. Consequently, the data flow must be orderly and organized. For example, the data acquisition modules transmit digital geophone array values in digital data packets containing a predetermined number of digital data bits. In addition to a seismic signal value, each of these data packets carries the identity of the specific geophone array from which the data originates and either explicitly or implicitly, the time it was received by the geophone. The acquisition modules are programmed to transmit a data packet respective to each geophone array channel at a predetermined frequency. The variable data in a data packet represents an instantaneous snapshot of the analog signal flow from the geophone array channel.
Managing an orderly flow of this massive quantity of data to a central recording unit requires a plurality of digital signal processing devices. The data acquisition modules convert the geophone analog data to digital data and transmit the digital data packets along receiver lines or radio transmission channels. There may be numerous data acquisition modules transmitting respective data packets along a single receiver line or channel. Among the functions of each data acquisition module is data packet transmission timing respective to the flow of data packets from other data acquisition modules transmitting respective data packets along the same receiver line. Typically, two or more receiver lines connect with base line units that further coordinate the data packet flow of numerous additional base line units into a base transmission line for receipt by a central recording unit.
Seismic surveying is often carried out under extremely inhospitable conditions of heat or cold, tropics or arctic, land and sea, desert or swamp. Regardless of the environment, the geophones must be positioned precisely relative to the seismic source event. Necessarily, manual placement of the geophones is required.
Due to the signal processing complexity of data acquisition modules and base line units, the “hard wired” electronic component assembly of these devices is expensive and relatively fragile. For this reason, the component assemblies are enclosed by protective environmental housings. There are as many different types of protective housings as there are environmental conditions. In some conditions, the housing must be waterproof to several hundred feet of depth, for example, but weight is of no great concern. In other conditions, mountains for example, the housing must be capable of withstanding severe shock as when falling from some height onto bare rock. Consequently, a world-wide seismic survey enterprise, in the past, has been required to maintain large inventories of seismic data acquisition equipment for rapid response for new surveys under diverse physical conditions.
As expensive as the protective housing may be, the cost of a housing is but a small fraction of the cost of the electronic component assembly that the housing protects. Moreover, to a great degree, the electronic component assembly in one type of protective housing is the same as in a different type of housing. Accordingly, such seismic survey groups must commit a large percentage of their equipment capital to replicate assemblies.
In other scenarios, the weight and volume of a protective housing is several times greater than the weight and volume of the electronic circuitry inside of the housing. Packing and preparing a seismic survey equipment assembly for delivery to a survey site is a large undertaking. Consequently, transport of a given survey equipment assembly from one survey site to another, distant, site is both, expensive and time consuming.